PYRENE DIMERIZATION IN CONTROLLED TEMPERATURE ENVIRONMENT: AN EXPERIMENTAL STUDY

Nucleation can be defined as the transition from gas phase to condensed phase state at flame temperature (the condensed phase state can be interpreted in many different ways from liquid-like to fully solid). This aspect of particle formation in flame is the least understood despite of the great effort of the last decades. Most of the models and of the experimental setup are not able to isolate the single step of nucleation. In general, the formation of the first particles or compounds in condensed phase is studied in combustion environments in which many other processes - namely PAH growth from other small hydrocarbons and oxidation - are taking places. The use of tubular flow reactor fed with a stream containing pure PAHs - pyrene in the current work - allows to isolate as much as possible the pathways leading from gas phase to condensed phase. In order to avoid the formation of smaller compounds from the pyrolysis of the pyrene, temperature has been kept not higher than 1000 K allowing to have a residence time in the tubular flow reactor long enough to obtain particle formation. Particles exiting from the reactor have been measured with a DMA obtaining the particle size distribution. The formation of particles has been found to be sensitive to the temperature. In particular, the formation rate was higher at the higher temperature, suggesting the intervention of some chemical-driven pathway from the formation of the first condensed phase compound. Successively the gas and the particles exiting the reactor were trapped in ethanol and analyzed by Size Exclusion Chromatography (SEC). The results confirmed the presence of compounds with molecular weight higher than 500 u and having no more a molecular-like absorption spectra, which can be considered the smallest compounds in condensed phase. Also the SEC showed the absence of intermediate compounds between these smallest compounds and pyrene, confirming these as the smallest compounds to be found from pyrene dimerization.